Well, I don't know if it is too late yet but in general I say do it! This is a great way to cut down on the cost of brewing. I just started a few months ago and have been reusing a few strains multiple times and really enjoying the hands on process of washing and reusing yeast.
I'm not going to tell you how to do it because in my research online I have found several good websites with alternate ways of going about it. Each totally acceptable and adaptable to the individual. Just do a search for washing yeast and you'll come up with a bunch of options. Pick the best one for you and go for it.
Scratch that, I've had a few too many homebrews and didn't read your post correctly. I thought you had the yeast cake in the carboy...not in the glass.
If it's been sitting out for a while I would dump it but that's me. I'm overly anal about reusing yeast and avoiding contamination.
I take mine from the primary. Here's the link I used: